1. 1 Dfdfdsa The Influence of Chemical Mechanisms on PDF Calculations of Nonpremixed Piloted Jet Flames Renfeng Richard Cao and Stephen B. Pope Sibley School of the Mechanical and Aerospace Engineering Cornell University, Ithaca, NY, 14853 This work is supported by Air Force Office of Scientific Research under grant No. F- 49620-00-1-0171 and the Department of Energy under Grant No. DE-FG02-90ER.

2. 2 Contents Introduction Introduction About turbulent combustion About turbulent combustion Experimental operating conditions Experimental operating conditions Calculations on piloted jet flames Calculations on piloted jet flames Joint PDF method Joint PDF method Tested mechanisms Tested mechanisms Numerical issues Numerical issues Comparison of different mechanisms Comparison of different mechanisms Sensitivity to reaction rates and the mixing model constant Sensitivity to reaction rates and the mixing model constant Conclusions Conclusions

3. 3 Why detailed chemistry calculations Turbulent combustion is important Turbulent combustion is important Research on turbulent combustion is difficult Research on turbulent combustion is difficult Simplified view of chemistry has been used for many years, which often show unacceptable limitations, such as the prediction of pollutant emissions or of stability limits Simplified view of chemistry has been used for many years, which often show unacceptable limitations, such as the prediction of pollutant emissions or of stability limits With the rapid increase of computer power and the development of efficient algorithms, turbulent combustion simulations with detailed chemistry have become more and more feasible in recent years. With the rapid increase of computer power and the development of efficient algorithms, turbulent combustion simulations with detailed chemistry have become more and more feasible in recent years.

4. 4 Why piloted jet flames Starner S.H. and R.W. Bilger, 1985 Starner S.H. and R.W. Bilger, 1985 Masri A.R., Bilger R.W. and Dibble R.W., 1988 Masri A.R., Bilger R.W. and Dibble R.W., 1988 Masri A.R., Dibble R.W. and Barlow R.S., 1996 Masri A.R., Dibble R.W. and Barlow R.S., 1996 Barlow, R.S., and J.H. Frank, 1998 Barlow, R.S., and J.H. Frank, 1998 A.N. Karpetis and R.S. Barlow, 2002 A.N. Karpetis and R.S. Barlow, 2002 Creating strong turbulence-chemistry interactions in a stable flame with relatively simple fluid mechanics and turbulence structure Demonstration of local extinction and reignition in these flames

5. 5 Introduction: Experimental operating conditions Dimensions: Nozzle diameter = 7.2mm Nozzle diameter = 7.2mm Pilot diameter = 18.2mm Main jet: Pilot diameter = 18.2mm Main jet: 25% CH 4 25% CH 4 75% air; 75% air; F stoic = 0.351 F stoic = 0.351 L vis ~ 67d Reynolds numbers: L vis ~ 67d Reynolds numbers: C-13400 C-13400 D-22400 D-22400 E-33600 E-33600 F-44800 F-44800

6. 6 Joint PDF calculations of piloted jet flames Previous work Previous work Xu, Pope, 2000, ARM1 mechanism, EMST Xu, Pope, 2000, ARM1 mechanism, EMST Tang, Xu, Pope, 2000, ARM2 mechanism, EMST Tang, Xu, Pope, 2000, ARM2 mechanism, EMST Lindstedt, Louloudi, Vaos, 2000, Lindstedt mechanism, MC Lindstedt, Louloudi, Vaos, 2000, Lindstedt mechanism, MC The current work The current work Six detailed and reduced mechanisms: GRI3.0 (53 species, 325 reactions), GRI2.11, ARM2, S5G211, Skeletal, Smooke Six detailed and reduced mechanisms: GRI3.0 (53 species, 325 reactions), GRI2.11, ARM2, S5G211, Skeletal, Smooke Tested flame: Flame F (and D and E) Tested flame: Flame F (and D and E) Autoignition Autoignition Laminar opposed-flow diffusion flame (OPPDIF) Laminar opposed-flow diffusion flame (OPPDIF)

7. 7 Joint PDF method TURBULENT COMBUSTION MODEL Joint velocity-turbulent frequency-composition PDF method Joint velocity-turbulent frequency-composition PDF method Software: HYB2D Developed by Muradoglu, Caughey, Pope, Liu and Cao Developed by Muradoglu, Caughey, Pope, Liu and Cao MIXING MODEL EMST (Euclidean Minimum Spanning Tree) CHEMICAL MECHANISMS GRI3.0, GRI2.11, ARM2, S5G211, skeletal, Smooke GRI3.0, GRI2.11, ARM2, S5G211, skeletal, Smooke ISAT PARALLEL ALGORITHM Domain partitioning of particles implemented using MPI

8. 8 Tested mechanisms Mechanism # of species # of steps NO speciesReferences GRI 3.053325With NOGRI-Mech Web site GRI 2.1149277With NOGRI-Mech Web site ARM21915With NO Sung et al., 1998 S5G21195With NOMallampalli et al., 1996 Skeletal1641Without NOJames et al., 1999 Smooke1646Without NOSmooke et al., 1986, Bennett

9. 9 Numerical Issues Calculation domain: Calculation domain: Statistically steady 2D axisymmetric Statistically steady 2D axisymmetric Inlet profiles Inlet profiles Implemented using the experimental data Implemented using the experimental data Numerical accuracy Numerical accuracy Statistical identical results of parallel and serial calculations Statistical identical results of parallel and serial calculations Numerical parameters that affect the accuracy of the results Numerical parameters that affect the accuracy of the results

10. 10 Convergence with respect to the ISAT error tolerance ISAT error tolerance is set to 2×10 –5 ISAT error tolerance is set to 2×10 –5 Less than 2% error for the test case Less than 2% error for the test case

11. 11 Numerical Accuracy Statistical identical results of parallel and serial calculations Statistical identical results of parallel and serial calculations ISAT (In Situ Adaptive Tabulation) error tolerance (2×10 –5 ) ISAT (In Situ Adaptive Tabulation) error tolerance (2×10 –5 ) The number of cells in the domain ( 96 by 96 ) The number of cells in the domain ( 96 by 96 ) The number of particles per cell (100) The number of particles per cell (100) The coefficients of the numerical viscosity ( 2 =0.25, 4 =2.0 ) The coefficients of the numerical viscosity ( 2 =0.25, 4 =2.0 ) The coefficients of time averaging (>2000 particle time steps with time averaging factor >600) The coefficients of time averaging (>2000 particle time steps with time averaging factor >600) Generally, < 2% error for mean major species, < 5% error in the minor species Significant statistical fluctuations can be observed in conditional rms’s downstream (which is not important for the current work).

12. 12 Results and discussions Introduction Introduction Experimental operating conditions Experimental operating conditions Calculations on piloted jet flames Calculations on piloted jet flames Joint PDF method Joint PDF method Tested mechanisms Tested mechanisms Calculation domain and boundary conditions Calculation domain and boundary conditions Numerical parameters Numerical parameters Results and discussion Results and discussion Calculation of the velocity field and mixture fraction Calculation of the velocity field and mixture fraction Comparison of different mechanisms Comparison of different mechanisms (1) Joint PDF calculations (2) Autoignition test (3) OPPDIF Sensitivity to the chemical reaction rates Sensitivity to the chemical reaction rates Sensitivity to the mixing model constant C φ Sensitivity to the mixing model constant C φ Conclusions Conclusions

13. 13 Velocity field Blue circles: measurements [Schneider et al.]; Red lines, PDF calculations using the GRI3.0 and the EMST mixing model with C φ =1.5 The calculated velocity profiles agree with the experimental data reasonably well The calculated velocity profiles agree with the experimental data reasonably well

14. 14 Mixture fraction Blue circles: measurements [Barlow et al.]; Lines, PDF calculations using the GRI3.0 and the EMST mixing model Red lines: C φ =1.5 Green lines: C φ =2.0 Increasing C φ does not always result in decreasing of rms mixture fraction at all locations Increasing C φ does not always result in decreasing of rms mixture fraction at all locations

15. 15 Effect of pilot temperature and comparison with previous calculations (z/D=15) z/D=15: most significant local extinction; Very sensitive to T p Red solid: HYB2D T p =1880 K Blue dash: PDF2DV T p =1880K Green dash dotted: PDF2DV T p =1860 K Black dots: measured HYB2D T p =1880 K PDF2DV T p =1860 K PDF2DV T p =1880K

16. 16 Comparison of mechanisms with z/D=15 Smooke: extinguished S5G211: highest conditional mean S5G211

17. 17 Comparison of mechanisms with autoignition and flame F (C φ =1.5) calculations S5G211: shortest IDT Smooke: longest IDT S5G211: highest conditional mean T Smooke: extinguished Flame F Autoignition

18. 18 Comparison of mechanisms with autoignition and flame F (C φ =2.0) calculations S5G211: shortest IDT Smooke: longest IDT Flame F Autoignition S5G211: highest conditional mean Smooke: lowest conditional mean

19. 19 OPPDIF Maxima against strain rates Maxima against strain rates Yellow: Smooke Yellow: Smooke Magenta: Skeletal Magenta: Skeletal Blue: GRI2.11 Blue: GRI2.11 Green: GRI3.0 Green: GRI3.0 Smooke: the smallest extinction strain rate Skeletal: overpredicts the CO and OH GRI3.0: has doubled level of NO than the GRI2.11

20. 20 Y CO |ξ Red: ARM Red: ARM Blue: GRI2.11 Blue: GRI2.11 Green: GRI3.0 Green: GRI3.0 Cyan: S5G211 Cyan: S5G211 Magenta: Skeletal Magenta: Skeletal The skeletal overpredicts CO for ξ>0.5 at z/D=30

21. 21 Y NO |ξ Red: ARM Red: ARM Blue: GRI2.11 Blue: GRI2.11 Green: GRI3.0 Green: GRI3.0 Cyan: S5G211 Cyan: S5G211 The GRI3.0 yields higher level of the NO by a factor of two compared to the GRI2.11 and ARM2 mechanisms

22. 22 Sensitivity of the reaction rates Smooke Blue: doubled reaction rates Blue: doubled reaction rates Red: tripled reaction rates Red: tripled reaction rates Get stable flame by doubled the reaction rates 1.9 times the reaction rates still extinguished

23. 23 Sensitivity of the reaction rates S5G211 Blue: standard reaction rates Blue: standard reaction rates Red: tenth reaction rates Red: tenth reaction rates The 5-step mechanism is not a good mechanism

24. 24 Sensitivity to the mixing model constant C φ C  =1.2 C  =1.5 C  =2.0 C  =3.0 More sensitive to the change when the calculations close to global extinction Skeletal ARM2 S5G211 GRI2.11 GRI3.0 Smooke z/D=15

25. 25 Maximal temperature against C  at z/D=15 Skeletal ARM2 S5G211 GRI2.11 GRI3.0 ARM2 S5G211 Skeletal GRI2.11 GRI3.0 Smooke Similar tendency for all mechanisms (horizontally shifted) More sensitive to the change when the calculations close to global extinction Measured

26. 26 Conclusions (1/2) The performance of six detailed and reduced mechanisms has been investigated using the joint PDF calculations of flame F The large number of numerically-accurate PDF calculations reported here demonstrates that this PDF/ISAT methodology can be effectively applied to turbulent flames using chemical mechanisms with of order 50 species. For different mechanisms, longer IDTs, smaller extinction strain rate (in OPPDIF), lower conditional mean temperature (in flame F) Sensitivities of these calculations to the reaction rates and the mixing model constant C φ has been studied. Generally, the closer to the global extinction, the more sensitive to theses parameters.

27. 27 Conclusions (2/2) The GRI and ARM mechanisms (GRI2.11, GRI3.0 and ARM2) yield comparable results in agreement with experimental data (except for NO) As previously observed, GRI3.0 overpredicts NO by a factor of 2 The 5-step mechanism under-predicts local extinction substantially The Smooke mechanism has longer IDT and over- predicts local extinction The skeletal mechanism is generally good but it over- predicts CO

28. 28 Thanks and questions Thank you for your attention! Open for questions or comments Thank you! Questions?